In the "Home Menu", click "Settings", then "Advanced" and then click "Code Injection".
Add your Plausible Analytics script to the "Header" text box. Code added into the "Header" field is injected into the
<head>tag on every page of your site.
After adding your code, click on the "Save" button.
Now you can go to your website and verify whether Plausible Analytics script has been added and to your Plausible Analytics account to see whether the stats are being tracked. See here how to verify the integration.